62,797 research outputs found
Image Analysis of a Vesicle to Calculate the Bending Modulus
The cell membrane is an essential component of living cells and the dynamics of the membrane will provide insight into how a biological cell reacts to mechanical strain. Membrane mechanics are important in a variety of cellular processes like secretion, trafficking, signaling, and storage. Giant unilamellar vesicles are a model system for cellular membranes since the major component of all membranes is a phospholipid bilayer. Giant unilamellar vesicles allow one to examine physicochemical processes that occur in all cellular membranes, such as fusion, budding, and fission in a more controlled fashion. Contour fluctuations of the vesicles are analyzed to calculate the bending modulus of the lipid bilayer, which will provide insight to the cell membrane’s rigidity. An image processing program was developed that traces the thermal fluctuations of the vesicle membrane through edge detection. Theory of spherical harmonics was then applied to calculate the elastic properties of the bilayer based on the measured fluctuations
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Determination of biomembrane bending moduli in fully atomistic simulations.
The bilayer bending modulus (Kc) is one of the most important physical constants characterizing lipid membranes, but precisely measuring it is a challenge, both experimentally and computationally. Experimental measurements on chemically identical bilayers often differ depending upon the techniques employed, and robust simulation results have previously been limited to coarse-grained models (at varying levels of resolution). This Communication demonstrates the extraction of Kc from fully atomistic molecular dynamics simulations for three different single-component lipid bilayers (DPPC, DOPC, and DOPE). The results agree quantitatively with experiments that measure thermal shape fluctuations in giant unilamellar vesicles. Lipid tilt, twist, and compression moduli are also reported
Transitions in spatial networks
Networks embedded in space can display all sorts of transitions when their
structure is modified. The nature of these transitions (and in some cases
crossovers) can differ from the usual appearance of a giant component as
observed for the Erdos-Renyi graph, and spatial networks display a large
variety of behaviors. We will discuss here some (mostly recent) results about
topological transitions, `localization' transitions seen in the shortest paths
pattern, and also about the effect of congestion and fluctuations on the
structure of optimal networks. The importance of spatial networks in real-world
applications makes these transitions very relevant and this review is meant as
a step towards a deeper understanding of the effect of space on network
structures.Comment: Corrected version and updated list of reference
High-resolution single-pulse studies of the Vela Pulsar
We present high-resolution multi-frequency single-pulse observations of the
Vela pulsar, PSR B0833-45, aimed at studying micro-structure, phase-resolved
intensity fluctuations and energy distributions at 1.41 and 2.30 GHz. We show
that the micro-pulse width in pulsars has a period dependence. Like individual
pulses, Vela's micro-pulses are highly elliptically polarized. There is a
strong correlation between Stokes parameters V and I in the micro-structure. We
show that the V/I distribution is Gaussian with a narrow width and that this
width appears to be constant as a function of pulse phase. The phase-resolved
intensity distributions of I are best fitted with log-normal statistics. Extra
emission components, i.e.``bump'' and ``giant micro-pulses'', discovered by
Johnston et al.(2001) are also present at 2.3 GHz. The bump component seems to
be an extra component superposed on the main pulse profile but does not appear
periodically. The giant micro-pulses are time-resolved and have significant
jitter in their arrival times. Their flux density distribution is best fitted
by a power-law, indicating a link between these features and ``classical''
giant pulses as observed for the Crab pulsar, (PSR B0531+21), PSR B1937+21 and
PSR B1821-24. We find that Vela contains a mixture of emission properties
representing both ``classical'' properties of radio pulsars (e.g.
micro-structure, high degree of polarization, S-like position angle swing,
orthogonal modes) and features which are most likely related to high-energy
emission (e.g. extra profile components, giant micro-pulses). It hence
represents an ideal test case to study the relationship between radio and
high-energy emission in significant detail.Comment: accepted for publication in MNRAS (11 pages, 10 figures
Physics of Interpulse Emission in Radio Pulsars
The magnetized induced Compton scattering off the particles of the
ultrarelativistic electron-positron plasma of pulsar is considered. The main
attention is paid to the transverse regime of the scattering, which holds in a
moderately strong magnetic field. We specifically examine the problem on
induced transverse scattering of the radio beam into the background, which
takes place in the open field line tube of a pulsar. In this case, the
radiation is predominantly scattered backwards and the scattered component may
grow considerably. Based on this effect, we for the first time suggest a
physical explanation of the interpulse emission observed in the profiles of
some pulsars. Our model can naturally account for the peculiar spectral and
polarization properties of the interpulses. Furthermore, it implies a specific
connection of the interpulse to the main pulse, which may reveal itself in the
consistent intensity fluctuations of the components at different timescales.
Diverse observational manifestations of this connection, including the moding
behavior of PSR B1822-09, the peculiar temporal and frequency structure of the
giant interpulses in the Crab pulsar, and the intrinsic phase correspondence of
the subpulse patterns in the main pulse and the interpulse of PSR B1702-19, are
discussed in detail. It is also argued that the pulse-to-pulse fluctuations of
the scattering efficiency may lead to strong variability of the interpulse,
which is yet to be studied observationally. In particular, some pulsars may
exhibit transient interpulses, i.e. the scattered component may be detectable
only occasionally.Comment: 28 pages, 2 figures. Accepted for publication in Ap
Modeling RR Tel through the Evolution of the Spectra
We investigate the evolution of RR Tel after the outburst by fitting the
emission spectra in two epochs. The first one (1978) is characterized by large
fluctuations in the light curve and the second one (1993) by the slow fading
trend. In the frame of a colliding wind model two shocks are present: the
reverse shock propagates in the direction of the white dwarf and the other one
expands towards or beyond the giant. The results of our modeling show that in
1993 the expanding shock has overcome the system and is propagating in the
nearby ISM. The large fluctuations observed in the 1978 light curve result from
line intensity rather than from continuum variation. These variations are
explained by fragmentation of matter at the time of head-on collision of the
winds from the two stars. A high velocity (500 km/s) wind component is revealed
from the fit of the SED of the continuum in the X-ray range in 1978, but is
quite unobservable in the line profiles. The geometrical thickness of the
emitting clumps is the critical parameter which can explain the short time
scale variabilities of the spectrum and the trend of slow line intensity
decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To
appear in "The Astrophysical Journal
A microscopic modeling of phonon dynamics and charge response in metallic BaBiO
We use our recently proposed microscopic modeling in the framework of linear
response theory to investigate the complete phonon dispersion, the phonon
density of states, certain phonon-induced electronic charge distributions and
charge fluctuations (CF's) for anomalous soft modes of metallic BaBiO in
its simple cubic phase where superconductivity with up to 32 K appears.
The theoretical approach already has been applied successfully to the cuprate
high-temperature superconductors (HTSC's), simple ionic crystals (NaCl, MgO)
and perovskite oxides (SrTiO, BaTiO). It is well suited for
materials with a strong component of ionic binding and especially for "ionic"
metals. In particular, the giant phonon anomalies related to the breathing
vibration of the oxygen as found experimentally in superconducting doped
BaKBiO, resembling those observed in the high
cuprates, are investigated. The origin of these anomalies is explored and
attributed to a strong nonlocal coupling of the displaced oxygen ions to CF's
of ionic type, essentially of the Bi6s- and Bi6p orbital. This points to the
importance of both of these states at the Fermi energy. Starting from an
ab-initio rigid ion model (RIM) we calculate the effect on the lattice dynamics
and charge response of the most important electronic polarization processes in
the material, i.e. CF's and dipole fluctuations (DF's). Taking into account
these electronic degrees of freedom in linear response theory, we obtain a good
agreement with the measured phonon dispersion and in particular with the strong
phonon anomalies.Comment: Additional comparison with the cuprate HTSC's. A slightly shorter
version has been published in PR
Giant Fluctuations Induced by Thermal Diffusion in Complex Liquids
The GRADFLEX experiment of ESA has shown that under microgravity conditions a stationary thermodiffusion process is accompanied by giant non-equilibrium fluctuations with size as large as the size of the sample. In the presence of small concentration gradients, the features of the non-equilibrium fluctuations can be described by means of linearized hydrodynamics. However, the linear models are not suitable to describe most cases of applicative interest, such as fluctuations induced by large gradients and under non-stationary conditions. Moreover, presently the investigation of non-equilibrium fluctuations has mainly involved single component fluids and binary mixtures, but recently transport processes in ternary mixtures have attracted increasing interest due to the experiments performed on the International Space Station in the framework of the DCMIX project of ESA. The Giant Fluctuations (NEUF-DIX) project of ESA will investigate non-equilibrium fluctuations during diffusive processes occurring in complex multi-component mixtures, where one of the components is a macromolecule, such as a polymer, a colloid or a protein. Important objectives will be the exploration of the features of the fluctuations under non-ideal conditions, such as large gradients, transient processes, and concentrated samples, and the understanding of how the fluctuations affect the interactions between macromolecules. The project involves the development of a dedicated facility, consisting of an array of shadowgraph optical instruments working in parallel, each one equipped with a thermal gradient cell. Here we outline the design concept of the facility and the results of performance tests performed on a breadboard to evaluate the suitability of the designed instrument to carry out scientific measurements of non-equilibrium concentration fluctuations in space
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